"that’s how the UK limits such crazy application numbers/admit rates. "
Limiting the app to 5 colleges is a piece of it, but the other key piece is that each of the colleges have fairly well defined admissions criteria. Ie - applicants must have a certain # of a certain type of scores for admissions, it’s not very holistic. Much more pure stat and merit based, more similar to the Chinese method but doesn’t rely on a single test.
The royal kids (William and Harry) didn’t go to St Andrews because it was necessarily their top choice; their scores weren’t sufficient to get them into Oxford or Cambridge and unlike US “selective” schools (which really aren’t necessarily selective regarding grades especially if you’re an athlete, legacy/donor or URM), it’s more difficult to buy an admission through donations or being famous in the UK.